Safety system for an implantable defibrillator

ABSTRACT

A defibrillator pulse generator for pectoral implant utilizing the metal case as an electrode and operative to supply unique patterns of monophasic, biphasic, or pairs of electrical pulses to the connected electrodes.

This is a divisional of application Ser. No. 08/854,862, filed Mar. 19,1992, now U.S. Pat. No. 5,376,103.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates, to implantable defibrillator systems, andparticularly to the electrodes and pulse generators used with suchsystems.

2. Description of the Prior Art

The departure of the heart from normal action to uncoordinated andineffectual contractions "fibrillation" can lead to death within minutesunless corrected. One method of treatment to restore the normal heartaction involves passing electrical current through the heart muscle. Theeffectiveness of such treatment is dependent on a number of factors,including the location of the electrodes used to apply the electricalcurrent, the shape of the electrodes, and the magnitude, timing, andwaveform of the current. While all these factors are significant, afundamental problem of all such electrical treatments arises from thefact that they all require large currents, several amperes to accomplishdefibrillation. And, because the heart muscle typically presents anelectrical impedance in the range of 40 to 100 ohms, signal amplitudesof several hundred volts are required to obtain the necessary current.The requirements for relatively high voltage and several-ampere currentscombine to place great importance on efficient, low-resistance electrodearrangements for delivering the defibrillation signal to the heart.Ideally the electrode would have no resistance itself and would beplaced directly against the heart muscle to avoid the voltage dropacross the tissue that surrounds the heart.

Various approaches to the optimal electrode have been attempted. Forexample, the epicardial-patch electrodes comprise conductive andrelatively large-area elements stitched directly onto the exterior ofthe heart itself. While this approach is satisfactory from theelectrical standpoint, the attachment of the electrodes requires a majorsurgical procedure, such as opening the chest cavity and exposing theheart, as depicted schematically in FIG. 1. Aside from the danger thatsuch surgery presents to all patients, many patients who require thistreatment are in such poor condition that this procedure presents anunacceptable risk. In situations where such radical surgery isinappropriate, other, less effective, electrode configurations have beenused. For example, the transvenous technique utilizes a conductingfilament threaded through an opening in a vein, and into the heartinterior. When the filament coils up in a heart chamber, ideally againstthe chamber wall, a relatively large-area contact to the cardiac musclecan be made. This approach requires that two such electrodes be used,one in the right-atrium (RA) position or in the nearby superior venacava (SVC) position, and the other placed at the right-ventricular-apex(RVA) position. Despite the fact that transvenous electrodes can beinserted with a relatively simple surgical procedure, they have aserious shortcoming. Because of the design constraints that permit themto be threaded through the blood vessels, they cannot be depended uponto make adequate contact with the interior wall of the heart, andtherefore they sometimes do not direct adequate current through asufficient portion of the heart-muscle volume to achieve defibrillation.

Another option is to combine a transvenous electrode with a subcutaneouspatch (SUB) in the fashion described in U.S. Pat. No. 4,817,608 toShapland, and in U.S. Pat. No. 4,953,551 to Mehra. This approachimplants, a shallow, just-under-the-skin conductive element ofappreciable area on the patient's left side to serve as an electrode, asillustrated in FIG. 2. Since the patch is not directly on the heart,current must pass through the intervening body tissue and fluid to reachthe heart. The resistance of the intervening tissue and fluid requiresthe application of a higher voltage to achieve the desired currentthrough the heart muscle, and the passage of the current through theintervening material may lead to patient discomfort. Additionally, whilethe surgical procedure for implanting the subcutaneous patch isrelatively minor compared to that required for implantation ofelectrodes directly against the heart muscle, it still presents somerisk to the patient. Although the subcutaneous-patch approach providesthe advantage of simpler and less risky surgery, the proximity of asubcutaneous patch to the body's surface leaves the electrode relativelyunprotected, and as a result, such electrodes have been subject toflexure and breakage from mishaps, and even from normal body motions.

Many patients have experienced ventricular fibrillation, or are likelyto experience it. These patients are best treated by a defibrillatorthat is implanted in the body. Because of the relatively high voltageand substantial currents involved in treatment, the size and weight ofthe implanted pulse generator (PG) is an important factor. The term PGis used to identify the single package or module that contains theentire implanted defibrillator system, excluding only its electrodes andassociated electrical leads. The package is usually a sealed housingmade of titanium, selected for its relatively light weight and corrosionresistance. The weight of the PG is normally in excess of 200 grams, orroughly half a pound. While electrical efficiency would be better servedwith pectoral implantation, the size and weight of the PG usuallyprecludes this location for cosmetic and comfort considerations, and themore spacious abdominal cavity is normally the chosen implantation site.This, of course, is in spite of the fact that PG implantation nearer theheart would result in a more compact system, with shorter leads.

Implantation of the PG nearer the heart provides the advantage of a moreefficient system which in turn allows the size of the PG to be reduced.PG implantation near the heart also permits various new electrodearrangements, which are the subject of the present invention. Inparticular, it permits use of the metallic PG housing as an electrode(hereinafter abbreviated as "CAN". This is, in a sense, a "free"electrode since the housing is required in any case. While use of the PGenclosure as an electrode is suggested in U.S. Pat. No. 4,727,877 toKallok, the resulting consequences were not addressed.

It is anticipated here that electrode use of the pectorally implanted PGhousing will be primarily an augmentation of present systems that employa catheter for one or more purposes. Implanting the PG involves surgerylittle more invasive than that required to implant a subcutaneous patch.Furthermore, it eliminates the troublesome requirement for tunnelingwires under the skin that accompanies the subcutaneous patch, and the PGis also not subject to crumbling and breakage.

It is possible to use the PG enclosure as an electrode in combinationwith electrodes of the prior art, such as the RVA, SVC andsubcutaneous-patch (SUB) electrodes. This facilitates the use ofsequential defibrillation pulses having differing spatial axes,demonstrated in the prior art to reduce the amount of energy needed fordefibrillation. Energy consumption is a vital concern since it isdirectly related to size and therefore also implantability. This isdiscussed in more detail by D. L. Jones, et al., "Internal CardiacDefibrillation in Man: Pronounced Improvement with Sequential PulseDelivery to Two Different Lead Orientations", Circulation, Volume 73,pages 484-491, March, 1986, and in their U.S. Pat. No. 4,548,203. Seealso, Saksena, U.S. Pat. No. 4,944,300, and Kallok, U.S. Pat. No.4,727,877, as well as Tacker, European Patent Application 0,095,726.

SUMMARY OF THE INVENTION

The invention provides system and electrode designs, configurations, andcurrent-application patterns that are simpler, less troublesome, morereliable, more efficient, and also are less risky to the patient, andless costly than those of the prior art.

One objective of the present invention is to accommodate pectoralimplantation of a PG, with the PG housing used as an electrode incombination with prior art electrode arrangements. One such arrangementis shown schematically in FIG. 3. This arrangement is more compact thanbefore, and has substantially shorter leads. Eliminating even a portionof the parasitic resistance in the leads (by shortening then) issignificant here because of the high peak currents required foreffective defibrillation. A platinum or other polarization-decreasingcoating for the titanium case has been found to be advantageous.

It will be appreciated that the use of the PG case as an electrode isnot possible for abdominal implantations. Use of the PG CAN electrode inan abdominal implant would cause severe and painful shock to thepatient. Aside from this, such an arrangement would cause an intolerableenergy waste because of the need to push large currents through thediaphragm and portions of the abdominal organs in order to reach theheart.

Another aspect of the present invention is to use the PG-housingelectrode both in lieu of the subcutaneous-patch electrode and as anaugmentation to it, providing either two, three, or four electrodes.Either case opens wide, new opportunities for a variety ofpulse-sequence and pulse-axis combinations, with the second termreferring to the spatial direction of the discharge, fixed by polarityand electrode choices.

In pectoral implantation of a PG, the entire PG exterior may be employedas an electrode. This provides a large electrode area, and hence a smallparasitic contact resistance. While the low contact resistance is adesirable goal, the system could pose a serious shock hazard to medicalpersonnel handling it before and during implantation. Also, thisarrangement would not allow steering the current in a desired direction.

The application of an insulating layer to portions of the PG's externalsurface largely eliminates the shock hazard and provides the beneficialresult of allowing the current to be steered in a direction mostadvantageous for defibrillation. The PG housing desirably approximates asomewhat flattened rectangular parallelpiped. This geometry allows mostof one major face of the housing to serve as the electrode, with thebalance being insulated, as is illustrated in FIGS. 4, 5 and 6. Becausethe four smallest faces, or edges, as well as one major face, of the PGare insulation-covered, safe handling of the PG is comparativelystraightforward and can be accomplished without risk to the surgeonduring implantation. A further benefit of this arrangement is that theelectrical discharge can be aimed in a chosen direction. For example,aiming the discharge toward the interior of the body causes primarycurrent conduction to avoid the skin, which largely avoids theadditional discomfort normally accompanying an electrode not in directcontact with the heart. On the other hand, aiming the discharge awayfrom the interior of the body causes the path length, and henceparasitic resistance, to increase, but causes less skeletal muscle"jerk". By this is meant a reflexive contraction of skeletal muscles inthe path of the electrical discharge, and stimulated by it, withuncomfortable, and possibly injurious results.

As a further option, another portion of the PG housing could be coveredwith an insulating coating, as shown in FIGS. 7 and 8. The ease andsafety of handling of this configuration approximates that for thepreceding option, but additionally provides a wider range of aimingoptions due to the increased number of surfaces which are not insulated.

While the conductive PG housing will be most advantageously used in thepectoral implant, it can also be used in conventional abdominalimplantation by adding a single-pole, single-throw selector switch tothe system, as shown in FIG. 9. When selector switch 94 is open, as inFIG. 9, the metal housing of the PG is isolated from all circuitry, andthe PG may be conventionally implanted in the abdominal cavity. But whenselector switch 94 is closed, the PG housing is activated as the CANelectrode. By the simple act of plugging in the lead from a SUBelectrode, and (or) an RA electrode, the surgeon can realize variouselectrode-pattern options to accompany the pectorally implanted CANelectrode.

In the event that further protection against shock is desired, thisinvention provides a circuit, shown in FIG. 10, for sensing that theimplantation procedure has not yet been performed and develops adisabling signal to prevent inadvertent generation of the defibrillationsignal. This feature totally eliminates the shock hazard to medicalpersonnel. It can be viewed as a safety element that augments theexterior insulation described above.

It is evident that combining the PG-housing or CAN electrode with thewell-established defibrillation electrodes SVC and RVA, that are oftenassociated with a cardiac catheter, makes possible a number of polaritypatterns for applying defibrillation pulses. Beyond this, is the choiceof the monophasic pulse pictured in FIG. 11, the biphasic pulse in FIG.12, and the sequential pulses in FIG. 13. Let it be said that the twopulses in the biphasic waveform, as well as in the sequential waveformare of comparable amplitude and duration, thus avoiding the infinity ofpossible waveform variations. Let "comparable" be taken to mean "withina factor of four".

Consider first the monophasic pulse. Taking the three electrodes in thesequence RVA, SVC, and CAN, FIG. 14 identifies four polarity patternsthat are useful. The number in the left-hand column identifies thepattern. The plus and minus symbols indicate relative polarities of therespective electrodes during discharge, and the zero symbol means thatthe circuit to the corresponding electrode is open, or else that theelectrode is otherwise omitted from the systems. It will be seen thatoptions assigning a zero to the RVA electrode are omitted, because theRVA electrode plays a dominant role in directing current through thebulk of the left-ventricular muscle. Furthermore, it has been found thatassigning the same polarity to the RVA and SVC electrodes, that is,making them electrically common, is an ineffective option. Note thatsimple polarity reversal has been treated as a separate pattern. Thatis, pattern 3 is the reverse of 1, and 4 is the reverse of 2. Finally,the case with the CAN electrode open or removed is omitted because itreverts to the prior art.

Next, the four patterns in FIG. 14 may be interpreted as a descriptionof the first pulse in the biphasic waveform of FIG. 12. Thus, FIG. 14deals fully with both the monophasic and biphasic cases. The case of twopulses in sequence involves additional considerations. First, identify agiven sequential-pulse option by using the pattern identificationnumbers. Thus, "12" would mean that the first pulse is of pattern 1, andthe second, pattern 2. It has been found that two same-pattern (andotherwise similar) pulses in a sequence are not beneficial. In thesequential-pulse representation of FIG. 13, different polarity patternsare assumed for the two pulses. Therefore, the sequence options 11, 22,33 and 44 are dropped from consideration. Next, a sequence involvingsimple polarity inversion on all electrodes in going from the firstpulse to the second is also omitted because this simply constitutes oneof the biphasic options. This removes 13, 31, 24 and 42. Next, considerthat a pattern eliminating the RVA electrode may be useful as one of thetwo sequential pulses, even though it is not useful in the monophasiccase. There are two such patterns given in FIG. 15, and numbered 5 and6. Thus, it is possible to list exhaustively all useful patterncombinations in the sequential case, as has been done in FIG. 16.

When a subcutaneous-patch or SUB electrode is present in addition to theRVA, SVC, and CAN electrodes, the list of patterns must be reconsidered.Once again, a pattern with RVA and SVC common is rejected for the samereason as before. Further, a pattern with CAN and SUB having oppositepolarities is rejected because the current from one to the other wouldbe remote from the heart and wasted. In addition, a pattern with CANopen is avoided because it constitutes prior art, and a pattern with SUBopen is also avoided because such cases have already been treated inFIGS. 14, 15 and 16. Thus, there are four patterns again this time, asgiven in FIG. 17. Again, there are two additional patterns that arepotentially useful in the sequential case, as given in FIG. 18. Becausethe symmetries in FIGS. 17 and 18 are identical to those in FIGS. 14 and15, it follows that FIG. 16 gives the useful pattern combinations forthe case of four electrodes, as well as for the case of threeelectrodes.

One significant aspect and feature of the present invention is animplantable pulse generator for defibrillation that is lighter inweight, as well as being smaller in size, than those of the prior art,and hence lends itself to pectoral implantation.

Another significant aspect and feature of the present invention is acompact defibrillation system having leads shorter than those of theprior art.

A further significant aspect and feature of the present invention isusing the PG's metal housing as an electrode without creating a hazardto medical personnel during implantation, nor undue discomfort to thepatient during the defibrillation process.

A further significant aspect and feature of the present invention is aPG metal housing designed to serve as an electrode, but which is partlycovered by an insulating layer that has the combined function ofproviding protection from discharges for medical personnel, who handlethe system before and during implantation, and of "steering" theelectrical current within the body.

Yet another significant aspect and feature of the present invention isthe addition of a selector switch to the PG of the invention that willpermit its use in a conventional abdominal implantation withconventional electrodes, as well as in the pectoral-implantation optionwherein the housing is an electrode.

An even further significant aspect and feature of the present inventionis a safety feature involving a comparator circuit that sensesmetal-housing-to-circuit-common resistance, and disables the PG unlessthat resistance is low enough to signify system implantation has beencompleted, further protecting medical personnel before and duringsurgical implantation.

Yet a further aspect and feature of the present invention is the use ofthe PG-metallic-housing (CAN) electrode, in lieu of or in combinationwith a subcutaneous-patch (SUB) electrode, and in combination with theRVA and SVC electrodes, to provide a wide range of polarity-pattern anddischarge-axis choices for monophasic and biphasic waveforms, as well asa larger number of pulse-pair combinations for use of the sequentialtechnique.

Having thus described the embodiments of the present :invention, it is aprincipal object hereof to provide a pulse generator for defibrillationthat is small enough in size and weight to be suitable for pectoralimplantation.

Another object of the present invention is to create an implantabledefibrillation system that is compact and comprises leads much shorterthan those of the prior art.

A further object of the present invention is to provide a defibrillationelectrode that is efficient, well-positioned, and "free" in the sensethat it is the CAN or housing of the pulse generator that must in anycase be present.

Yet another object of the present invention is to protect medicalpersonnel from hazardous shocks during implantation by partly coveringthe housing electrode by an insulating layer.

Yet a further object of the present invention is an ability to steer thedefibrillating current within the body by insulating selected portionsof the housing electrode.

Another object of the present invention is versatility inpulse-generator design, with a selector switch being able to convert itfrom a module suitable for pectoral implantation with the housing as anelectrode to a module suitable for conventional abdominal implantations.

A related object of the present invention is to provide a wide range ofpolarity pattern and discharge-axis options for monophasic and biphasicdefibrillation waveforms, as well as a larger number of pulse-paircombinations for use of the sequential technique.

A further object of the present invention is a safety provision formedical personnel before and during implantation in the form of acomparator circuit that senses output resistance and disables the pulsegenerator unless that resistance is low enough to signify thatimplantation has been completed.

Yet another object of the present invention is to provide severalpolarity-pattern and discharge-axis options by combining the PG-housingelectrode with more conventional electrodes such as the RVA and SVCcoils on a standard catheter.

Yet a further object of the present invention is to replace thesubcutaneous-patch electrode by a PG-housing electrode that eliminatesthe need for tunneling wires under the skin and eliminates the hazardsof breakage and crumbling that accompany the subcutaneous-patch option.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects of the present invention and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, in which like reference numerals designate like partsthroughout the figures thereof and wherein:

FIG. 1 illustrates a schematic representation of a defibrillating systemof the prior art implanted in the abdominal cavity, and havingepicardial-patch electrodes attached directly to the heart;

FIG. 2 illustrates a schematic representation of a defibrillating systemof the prior art having one transvenous electrode and onesubcutaneous-patch electrode;

FIG. 3 illustrates a schematic representation of a defibrillating systemof the present invention having a SVC electrode, an RVA electrode andone subcutaneous-patch electrode;

FIGS. 4, 5 and 6 illustrate schematic representations of adefibrillating system of the present invention having a PG housing withone major metallic face exposed to serve as an electrode, and thebalance of the PG surface area covered by an insulating layer;

FIGS. 7 and 8 illustrate schematic representations of a defibrillatingsystem of the present invention having a PG housing with approximatelyhalf its surface area exposed to serve as an electrode, and the balanceof the PG surface area covered by an insulating layer;

FIG. 9 illustrates a schematic representation of a defribillating systemof the present invention incorporating a selector switch that permitsthe PG to serve either in the PG-housing-as-electrode mode or in otherconventional modes;

FIG. 10 illustrates a schematic representation of a defibrillatingsystem of the present invention incorporating one possible safetycircuit that disables the pulse generator when thehousing-to-circuit-common resistance is higher than that encountered bythe system after implantation, thus protecting medical personnel whomust handle the system before and during implantation;

FIGS. 11 illustrates a monophasic waveform that in the present inventionis applied to a novel set of electrodes in novel patterns;

FIG. 12 illustrates a biphasic waveform that in the present invention isapplied to a novel set of electrodes in novel patterns;

FIG. 13 illustrates a sequential-pulse waveform that in the presentinvention is applied to a novel set of electrodes in novel patterns;

FIG. 14 illustrates a chart of useful polarity patterns for threeelectrodes, RVA, SVC, and CAN, describing the cases of monophasic andfirst-biphasic-pulse waveforms;

FIG. 15 illustrates a chart of additional polarity patterns for use insequential-pulse waveforms in the three-electrode case;

FIG. 16 illustrates a chart of three- and four-electrode patterncombinations useful in sequential-pulse defibrillation;

FIG. 17 illustrates a chart of useful polarity patterns for fourelectrodes, RVA, SVC, CAN, and SUB, for the cases of monophasic andfirst-biphasic-pulse waveforms; and,

FIG. 18 illustrates a chart of additional polarity patterns for use insequential-pulse waveforms in the four-electrode case.

DESCRIPTION OF THE PRIOR ART

FIG. 1 illustrates a schematic drawing of a patent 10 fitted with adefibrillating system of the prior art consisting of a PG 12 implantedin the abdominal cavity and connected to epicardial-patch electrodes 14and 16 by electrical-lead harness 18.

FIG. 2 illustrates a schematic drawing of a patient 20 fitted with adefibrillating system of the prior art consisting of a PG 22 implantedin the abdominal cavity and connected to transvenous RVA electrode 24and subcutaneous-patch electrode 26 by means of electrical-lead harness28 where all numerals correspond to those elements previously described.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 illustrates a schematic drawing of a patient 30 fitted with adefibrillating system of the present invention comprising a pectorallyimplanted PG 32, a subcutaneous-patch electrode 34, and transvenouscatheter 36, carrying an SVC electrode 38, and an RVA electrode 39 whereall numerals correspond to those elements previously described.

FIG. 4 illustrates the top face 40 of the PG 42 having an insulatinglayer 44 that covers the entire top surface of the PG exterior where allnumerals correspond to those elements previously described.

FIG. 5 illustrates an elevation of a PG 42 having an insulating layer 52that covers the entire surface of the face 50 depicted, and also coversthe remaining three "edge" faces where all numerals correspond to thoseelements previously described.

FIG. 6 illustrates the bottom face 60 of the PG 42 having an insulatinglayer 62 that covers only the periphery of the bottom major face 60,leaving the balance 64 of the bottom face 60 within the periphery of theinsulating layer 62 to serve as an exposed-metal electrode.

FIG. 7 illustrates a side view of a PG 72, including a plurality offaces 70a-70n, having an insulating layer 74 that: covers a significantfraction of the exterior surface of the PG 72, leaving the balance 76consisting of faces 70a-70n of the exterior surface of the PG 72 in theform of exposed metal to serve as an electrode.

FIG. 8 illustrates a top view of the PG 72 and the insulating layer 74that covers a significant fraction of the faces 70a-70n, leaving thebalance 84 consisting of faces 70a-70n in the form of exposed metal toserve as an electrode.

FIG. 9 illustrates a PG module 90 and represents schematically certainof its internal elements that permit flexible application of the systemwhere all numerals correspond to those elements previously described.The pulse-generator circuit 92 has a first output lead 93 connectedthrough the externally controlled SPST selector switch 94 to the PGhousing 95 at the connection point 96. When the switch 94 is open, thePG module 92 can be abdominally implanted in conventional fashion; whenthe switch 94 is closed, the PG housing 95 can be employed as adefibrillation electrode in the case of pectoral implantation. The firstoutput lead 93 is also connected to a first self-sealing output jack 98into which an SVC electrode lead can be plugged when desired, as well asto a second self-sealing output jack 100 into which a SUB electrode canbe plugged when desired. A second output lead 101 from thepulse-generator circuit 92 is permanently connected inside a lead 102that is intravenously installed to place an electrode in the RVAposition. Activation of an SVC electrode is accomplished by plugging itslead into jack 98, and activation of a SUB electrode is accomplished byplugging its lead into jack 100. With these options, in addition to thatprovided by selector switch 94, it is evident that the flexibility ofthe present invention offers the choice of three single-electrodeoptions, of three common-double-electrode options, and onecommon-triple-electrode option, for a total of seven options for anelectrode pattern to deliver a shock directed at the opposing RVAelectrode that is connected to the pulse-generator circuit 92 throughthe lead 102.

FIG. 10 illustrates a PG module 110 that incorporates a safety circuitfor disabling the pulse generator until the system has been implantedwhere all numerals correspond to those elements previously described.The safety circuit senses when the system has been implanted bymonitoring the resistance between the implanted RVA electrode 134 andthe metal housing 130 of the system. When the resistance drops to a lowlevel, the system develops a signal that allows defibrillation pulses tobe passed to the CAN or PG-housing electrode.

When the pulse generator 140 is prepared to deliver its pulse or otherwaveform it closes SPST switch 112 by conventional circuit means.Closing SPST switch 112 causes current from low-voltage power supply 114to flow through a center-tapped 1-megohm resistor, that is throughresistors 116a and 116b. This creates a reference voltage, having avalue one half that of the output from the low-voltage supply 114, to bedeveloped across resistor 116a, and causes the centertap 118 to become areference terminal.

The reference voltage at the centertap 118 is fed to a first, positive,input terminal 120 of comparator 122. A "test" voltage, responsive tothe resistance between the CAN electrode metal housing 130 and the RVAelectrode 134 is applied to a second, negative, input terminal 124 ofcomparator 122. This voltage is derived from a voltage dividerconsisting of a 500-ohm resistor 126 as the "upper" element, and as the"lower" element, the resistance 128 existing at that time from the metalhousing of the PG or CAN electrode 130 to the common terminal 132 of thehigh- and low-voltage circuits, which is also common to the RVAelectrode 134. It will be appreciated that, while FIG. 10 illustratesthe resistance between the CAN electrode 130 and the RVA electrode 134as a resistor 128 shown in dotted lines, in actuality, the resistance isnot a discrete resistor, but rather the resistance of the path thatexists at the time between these electrodes. Before the device isimplanted, the path will be largely air and have a very high resistance.However, after implantation, the path will be through relatively highlyconductive body tissue, and therefore, have a relatively low resistance.

Even when a person is handling the system, and holding the metal housingof the system in one hand and the RVA electrode in the other, theresistance between circuit points 130 and 134 (from hand to hand) istypically several kilohms, so that the test voltage at negative inputterminal 124 is much more positive than the reference voltage atpositive input terminal 120, so that the comparator delivers a logical"low", or zero voltage at output terminal 136. This output signalcontrols the switch 138, and zero voltage to that switch, which ispreferably an FET, meaning that the switch is inactivated and henceopen. With switch 138 open, the defibrillation pulses from pulsegenerator 140 are blocked and do not reach the CAN or housing electrode130.

When the PG module 110 is properly implanted, the electrical pathrepresented by the resistor 128 from the housing electrode 130 to theRVA electrode 134 will lie through body tissue and have a resistancevalue well below 500 ohms, causing the reference voltage at positiveinput terminal 120 to be more positive than the test voltage at negativeinput terminal 124, causing the comparator to switch to the logical"high" condition at output terminal 136. The high signal at comparatoroutput terminal 136 causes switch 138 to close, thereby permitting thenormal delivery of the defibrillation pulses from pulse generator 140 tothe metal housing 130.

The safety circuit operates for all CAN electrode 130 configurationswithout modification and functions to prevent accidental shockregardless of the selected pulse polarity. Thus, the medical team isprotected in all situations where the shock hazard is present and thesafety feature imposes no limitations on the electrode selection, thechoice of pulse polarity, or other options such as the pulse sequence orwaveform. Further, it is evident that the PG module 110 and itscircuitry of FIG. 10 can be combined with the PG module 90 and itscircuitry of FIG. 9 by combining the switches 138 and 94 into one switchoperable by either of two means.

FIG. 11 illustrates a defibrillation waveform 150 known in the prior artas monophasic that in the present invention is applied to a novel set ofelectrodes in novel patterns.

FIG. 12 illustrates a defibrillation waveform 160 known in the prior artas biphasic that in the present invention is applied to a novel set ofelectrodes in novel patterns.

FIG. 13 illustrates a defibrillation waveform 170 comprising a pair ofsequential pulses that in the present invention is applied to a novelset of electrodes in novel patterns.

FIG. 14 illustrates a chart set 180 of useful polarity patterns fordefibrillation using three electrodes: right-ventricular apex (RVA);superior vena cava (SVC); and PG housing (CAN). The set 180 omitspatterns that have been found ineffective. The plus and minus symbolsindicate relative polarities of the respective electrodes duringdischarge, and the zero symbol means that the circuit to thecorresponding electrode is open, or that the corresponding electrode isotherwise removed from the system. The set 180 is applicable to amonophasic waveform, and to the initial pulse of a biphasic waveform.

FIG. 15 illustrates a chart set 190 of additional polarity patterns fordefibrillation using the RVA, SVC and CAN electrode patterns that arefor use in one of the pulses in a two-pulse sequential waveform.

FIG. 16 illustrates a chart set 200 of twenty-four pattern combinationsfor use in sequential-pulse defibrillation. Each digit in the chartrefers to the corresponding polarity pattern defined in FIGS. 14 and 15,and each pair of digits represents a sequential-pulse option for twopulses in the case of three electrodes as in FIGS. 14 and 15, and forthe case of four electrodes as in FIGS. 17 and 18 which follow.

FIG. 17 illustrates a chart set 210 of useful polarity patterns fordefibrillation using the RVA, SVC, CAN, and SUB (subcutaneous-patch)electrodes. The set 210 omits patterns that are known to be ineffective,and is applicable to a monophasic waveform, and to the initial pulse ofa biphasic waveform.

FIG. 18 illustrates a chart set 220 of additional polarity patterns fordefibrillation using the RVA, SVC, CAN and SUB electrodes, patterns thatare for use in one of the pulses in a two-pulse sequential waveform.

Various modifications can be made to the present invention withoutdeparting from the apparent scope hereof.

The upper heart electrode can be at other locations, such as locationsof the right atrium and coronary sinus.

We claim:
 1. A safety system for an implantable defibrillation systemcomprising:a. defibrillation pulse-generating means having first andsecond output terminals; b. first electrode means comprising a metalliccase enclosing said means for generating defibrillation pulses; c.safety switch means connecting said first output terminal to said firstelectrode means; d. second implantable electrode means; e. circuit meansconnecting said second output terminal to said second electrode means;f. circuit means for measuring the resistance between said first andsecond output terminals and generating a pulse-disable signal when saidresistance is above a predetermined value representing the resistance ofan implanted system and generating a pulse-enable signal when theresistance between said first and second terminals is below saidpredetermined value; and, g. safety switch means responsive to saidpulse-enable and pulse-disable signals to connect said first terminal tosaid first electrode means in response to said pulse-enable signal andto disconnect said first terminal from said first electrode in responseto said pulse-disable signal.
 2. The safety system according to claim 1wherein said circuit means for measuring the resistance comprises:a.comparator means having first and second input terminals and an outputterminal; b. reference-voltage-generating means for generating areference voltage at said first comparator input terminal; and, c.test-voltage-generating means for generating a test voltage, responsiveto the resistance between said first and second output terminals, atsaid second comparator input terminal whereby said comparator operatesto generate a defibrillation pulse-enable signal when said test voltageis less than said reference voltage and to develop a defibrillationpulse-disable signal when said test voltage is greater than saidreference voltage.
 3. The safety system according to claim 2 whereinsaid reference voltage is a measure of the resistance between said firstand second terminals corresponding to a non-implanted device.
 4. Thesafety system according to claim 3 wherein the resistance represented bysaid reference voltage is above that of an implanted device.